Growth hormone releasing hormone (GHRH) is secreted by the hypothalamus, and stimulates the release of growth hormone (GH) from the anterior pituitary. GHRH is a member of a family of homologous peptides that includes glucagon, secretin, VIP (vasoactive intestinal peptide), PHI (peptide histidine isoleucine), PACAP (pituitary adenylate cyclase activating peptide), GIP (gastric inhibitory peptide), and helodermin. GHRH has been the subject of considerable study, but little is known about the GHRH receptor, GHRH-R, to which GHRH binds in the anterior pituitary to induce the release of GH.
Large scale production of the cloned GHRH receptor would enable the screening of large numbers of GHRH analogs, and would facilitate the development of improved agonists and antagonists in the clinical therapy of growth disorders. More specifically, the screening of large numbers of analogs and xenobiotics for GHRH activity could lead to the development of improved agonists for use in clinical therapy of growth hormone deficient children, and in clinical therapy of adults to improve nutrition and to alter body composition (muscle vs. fat). Such screening, possibly assisted by computer modeling based on receptor structure, could also lead to orally active non-peptide GHRH agonists that would be especially useful in medical and veterinary practice, for example, to develop higher yield milk production and higher yield, leaner livestock.
Commerical exploitation of drugs which interact with the GHRH-R will require a source of a purified form of GHRH-R and suitable binding assays.
The isolation and cloning of the GHRH receptor and its in vitro expression will also lead to: (1) In situ hybridization studies mapping the distribution of GHRH receptors throughout the body and to examination of their potential physiological role outside the pituitary; this might reveal potential roles for GHRH in the brain, gonad, pancreas, placenta, and gut, where the peptide is thought to be concentrated. (2) Studies of receptor structure involving mutated or chimeric receptors to explore structure/function relations and second messenger interactions in the quest for specifically tailored agonist/antagonist molecules. (3) An understanding of the GHRH-R's evolutionary relation to other G-protein-linked receptors, especially those in the glucagon/secretin/VIP family. (4) Cloning of other members of this sub-family that are expected to have sequence similarity.
There are several alternate routes towards obtaining functional receptor clones, such as: A. Purification of the receptor protein to obtain a partial protein sequence; this partial protein sequence could then be used to screen appropriate DNA for the corresponding nucleotide sequence. B. screening DNA for sequences similar to known receptors thought to be related to the GHRH receptor. C. Screening for DNA that, when expressed as protein, yields GHRH receptor, which would be detected by GHRH binding, biological activity, or by a GHRH receptor antibody. Note that any conceivable cloning method requires binding assays as well as functional assays with GHRH and related peptides in order to identify the GHRH receptor and to characterize expressed clones.
There is thus a need for GHRH binding assays and compositions for use therein which will help characterize and isolate the GHRH receptor. In particular, there is a need for methods which can characterize the pituitary GHRH receptor in terms of size, glycosylation, solubility and stability of the receptor (GHRH-R)--ligand (GHRH or GHRH analog) complex, so that methods can be developed to purify the receptor protein and identify receptor clones. There is also a need for purified or partially purified GHRH-R and methods for obtaining same. By partially purified GHRH-R, it is meant that a GHRH-R isolate is formed having GHRH-R isolated from most of the organic matrix of the anterior pituitary cells. The GHRH-R isolate has a purity sufficient to allow for determining the GHRH-R sequence, with it being understood that this may require further purification to remove any remaining compounds which would interfere with sequencing, such as G-proteins. Nevertheless, the GHRH-R isolate of the present invention, produced using the extraction and isolation method of the present invention contains GHRH-R, preferably at a concentration greater than that at which GHRH-R is naturally present in the anterior pituitary, and the GHRH-R isolate can be further purified, if necessary, using SDS-PAGE to remove any compounds which would interfere with sequencing of the GHRH-R. Thus, the present invention also includes the production of GHRH-R of sufficient purity to conduct sequencing, or for use in bioassays of GHRH-R binding activity.
Historically the GHRH-R has been difficult to work with since GHRH has very high non-specific binding (making it difficult to determine whether or not GHRH or GHRH analogs are binding specifically to GHRH-R), and GHRH-R has extremely low abundance. The nonspecific binding of GHRH analogs to glass and plasticware has been a major problem in previous work limiting the accuracy and reproducibility of receptor binding studies. Because of the "sticky" nature of the negatively charged GHRH peptide, the use of a simple filtration type binding assay has been impossible. Nonspecific counts are so high as to preclude detection of specific binding. In addition, the commonly used blocking agent polyethylenimine, which is used for blocking nonspecific binding of proteins on glass fiber filters is positively charged, and will bind GHRH analogs (negatively charged) nonspecifically. Further, a soluble receptor preparation with high binding affinity, which would vastly enhance efforts to purify the GHRH-R, has not been available.
Prior studies have characterized the GHRH receptor with respect to it's affinity for probes, such as GHRH and related peptides, and linkage to G-protein. Attempts have also been made to use non-specific chemical cross-linkers to label the GHRH receptor. See, for example, Zysk, et al., "Cross-Linking of a Growth Hormone Releasing Factor-Binding Protein in Anterior Pituitary Cells," J. Biol. Chem., 261:1678 (1986), and Velicelebi, et al., "Covalent Cross-Linking of Growth Hormone-Releasing Factor to Pituitary Receptors," Endocrinology, 118:1278 (1986). The results of these two studies suggest, respectively, the presence of a 26 KDa and a 70 KDa GHRH-receptor in the anterior pituitary. The discrepancy between the molecular weight found in these two studies emphasizes the difficulties involved in isolating and characterizing the GHRH-R, and the need for improved methods and compositions useful for isolating and characterizing the GHRH-R.
GHRH binding to the rat anterior pituitary is believed to be influenced by GTP, which causes the GHRH-receptor to reduce its affinity for GHRH (GTP is said to uncouple the G protein GHRH-receptor complex). The high affinity state of GHRH-R bound to GHRH is believed to be stabilized by interactions with a guanine nucleotide regulatory protein to form a hormone-receptor-G-protein ternary complex. GTP is hypothesized to destabilize the G-protein-receptor interactions, resulting in dissociation of the GHRH/GHRH-R-G-protein complex and reversion of the independent receptor to a low affinity state, while the liberated G-protein goes on to activate its respective second messenger system. See Struthers, et al., "Nucleotide Regulation of Growth Hormone-Releasing Factor Binding to Rat Pituitary Receptors," Endocrinology, 124:24-29 (1989).
It has been discovered that, in ovine and bovine anterior pituitary tissues, GHRH and its analogs are displaced by 500 to 1,000 fold lower concentrations of GHRHa than VIP or PACAP. This finding is complementary to binding properties noted in the human pancreas (a source of secretin and VIP receptors) where the ability to stimulate adenylate cyclase in the presence of GTP shows an order of potency of secretin&gt;helodermin&gt;PHI.gtoreq.VIP&gt;GHRH(1-27)NH.sub.2. Similarly, using .sup.125 I-secretin, Kds obtained were secretin 0.8 nM, helodermin 200 nM, PHI 250 nM. VIP and GHRH(1-29)-NH.sub.2 induce only 20% inhibition at 10 .mu.M.
At supraphysiologic doses, GHRH is known to act at VIP receptors, and conversely VIP is a weak GHRH agonist.
Other articles which provide background information on isolation and characterization of hormone receptors include: Christophe, et al., "The VIP/PHI/secretin-helodermin/helospectin/GRH Family: Structure-Function Relationship Of The Natural Peptides, Their Precursors And Synthetic Analogs As Tested in vitro On Receptors And Adenylate Cyclase In A Panel Of Tissue Membranes," in Peptide Hormones As Prohormones: Processing, Biological Activity, Pharmacology, Ed. Jean Martinez, Pub. Ellis Horwood Lim. 1989, Chichester, England. Baburthe, et al., "Molecular Analysis of Vasoactive Intestinal Peptide Receptors: A Comparison With Receptors for VIP Related Peptides," Ann NY Acad. Sci., 527:296-313 (1988). Frohm, et al., "Growth Hormone-Releasing Hormone," Endocr Rev., 7:223-253 (1986). Seifert, et al., "Growth Hormone-Releasing Factor Binding Sites In Rat Anterior Pituitary Membrane Homogenates: Modulation By Glucocorticoids," Endocrinolgy, 117:424-426 (1985). Bilezikjian, et al., "Desensitization To Growth Hormone-Releasing Factor (GRF) Is Associated With Down-Reguolation of GRF-Binding Sites," Endocrinology, 118:2045-2052 (1986). Ishihara, et al., "Functional Expression and Tissue Distribution of a Novel Receptor for Vasoactive Intestinal Polypeptide," Neuron, 8:811-819 (1992). Ishihara, et al., "Molecular Cloning and Expression of a cDNA Encoding the Secretin Receptor," EMBO J, 10:1635-1641 (1991). Lin, et al., "Expression Cloning of an Adenylate Cyclase-Coupled Calcitonin Receptor," Science, 254:1022-1024 (1991). Juppner, et al., "A G Protein-Linked Receptor For Parathyroid Hormone and Parathyroid Hormone-Related Peptide," Science, 254:1024-1026 (1991). Frohman, et al., "Tissue Distribution and Molecular Heterogeneity of Human Growth Hormone-Releasing Factor in the Transgenic mouse," Endocrinology, 127:2149-2156 (1990). Paul et al., "Characterization of Receptors for Vasoactive Intestinal Peptide Solubilized From the Lung," J. Biol. Chem., 262:158-162 (1987). Guijarro et al., "Solubilization of Active and Stable Receptors for Vasoactive Intestinal Peptide from Rat Liver," Regulatory Peptides, 25:37-50 (1989). Cronin et al., "Biological Activity of a Growth Hormone Releasing Factor Secreted By a Human Tumor," Am. J. Physiol., 244 (Endocrinol Metab) E346-E353 (1983). Leong et al., "Enumeration of Lactotropes and Somatotropes in Cultured Male and Female Pituitary Cells: Evidence in Favor of a Mammosomatotrope Subpopulation," Endocrinology, 116:1371-1378 (1985). Munson, et al., "Ligand: a Versatile Computerized Approach For Characterization of Ligand-Binding Systems," Anal. Biochem., 107:220-239 (1980). Wessel, et al., "A Method for the Quantitative Recovery of Protein in Dilute Solution in the Presence of Detergents and Lipids," Anal. Biochem., 138:141-143 (1984). Bagnato et al., "Gonadotropin-Induced Expression of Receptors for Growth Hormone Releasing Factor in Cultured Granulosa Cells*," Endocrinology, 128, 2889-2894 (1991) (compositions studied by Bagnato et al show GHRH binding properties which are different from binding properties of pituitary tissues). All articles and other documents mentioned herein are incorporated by reference as if reproduced in full below.
While the foregoing studies have been helpful in developing a preliminary understanding of the behavior of the GHRH-R, there remains a need for a sensitive and reproducible assay for the GHRH-R, which will enable the further characterization of the GHRH-R leading to the purification and cloning of the GHRH-R. Such an assay must overcome the problems of nonspecific binding of GHRH and GHRH analogs, and the low abundundance of the GHRH-receptor. Iodination and purification of GHRH analogs with resultant high specific activity allows for the improvement of specific binding to crude anterior pituitary membranes.
Therefore it is a primary object of the present invention to develop a sensitive and reproducible assay for GHRH binding.
It is a further object of the present invention to develop reagents to specifically and unambiguously label the GHRH-receptor.
It is yet another object of the present invention to develop a GHRH-receptor purification scheme and to obtain a GHRH-R isolate of sufficient purity, or capable of being readily purified to a purity, which will allow for at least partial sequencing of GHRH-R.
Thus, it is a further object of the present invention to produce purified GHRH-R.